The present invention pertains generally to ophthalmic laser surgery procedures. More particularly, the present invention pertains to laser surgical procedures which are performed to reshape or restructure the cornea of an eye by using photoablation techniques to remove stromal tissue. The present invention is particularly, but not exclusively, useful as a method and system for creating a flap in the cornea of an eye that can be moved or lifted to expose stromal tissue for photoablation.
Within the past number of years, the so-called LASIK procedure has been used successfully to correct vision difficulties for a significantly large number of patients. In overview, a LASIK procedure is used to reshape or restructure the cornea of an eye in order to change its refractive properties. The object is to thereby minimize optical aberrations and to improve a patient""s vision by altering the corneal shape.
As is well known to those skilled in the art, a LASIK procedure involves the removal of stromal tissue by photoablation. For a typical LASIK procedure, this photoablation is accomplished using an xe2x80x9cexcimerxe2x80x9d laser. Excimer lasers, however, are most effective when they are used to superficially photoablate tissue. Accordingly, when using an excimer laser for the photoablation of tissue, it is necessary to somehow expose the target tissue that is to be photoablated. In the case of a LASIK procedure, it has been the practice to mechanically access the target tissue. Heretofore, this has involved the creation of a corneal flap which can be moved, or lifted, to expose the target tissue. The xe2x80x9cexcimerxe2x80x9d laser is then used to photoablate the exposed stromal tissue. After the photoablation of tissue is accomplished, as desired the flap can be repositioned over the stroma. A major benefit of this so-called xe2x80x9cFlap and Zapxe2x80x9d procedure is that trauma to the epithelial layer at the anterior surface of the cornea is minimized. Trauma to the stroma under the epithelial layer, however, may still be significant.
A general knowledge of the anatomy of the cornea of an eye is helpful for appreciating the problems that must be confronted whenever a corneal flap is created. More specifically, the cornea comprises various layers of tissue which are structurally distinct. In order, going in a posterior direction from outside the eye toward the inside of the eye, the various layers in a cornea are: an epithelial layer, Bowman""s membrane, the stroma, Decimet""s membrane, and an endothelial layer. Of these various structures, the stroma is the most extensive and is generally around four hundred microns thick.
In detail, the stroma of the eye is comprised of around two hundred identifiable and distinguishable layers of lamella. Each of these layers of lamella in the stroma is generally dome-shaped, like the cornea itself, and they each extend across a circular area having a diameter of approximately six millimeters. Unlike the layer that a particular lamella is in, each lamella extends through a shorter distance of only about one tenth to one and one half millimeters. Thus, each layer includes several lamellae. Importantly, each lamella includes many fibrils which, within the lamella, are substantially parallel to each other. The fibrils in one lamella, however, are not generally parallel to the fibrils in other lamellae. This is so between lamellae in the same layer, as well as between lamellae in different layers. Finally, it is to be noted that, in a direction perpendicular to the layer, the individual lamella are only about two microns thick.
Within the general structure described above, there are at least three important factors concerning the stroma that are of interest insofar as the creation of a corneal flap is concerned. The first of these factors is structural, and it is of interest here because there is a significant anisotropy in the stroma. Specifically, the strength of tissue within a lamella is approximately fifty times the strength that is provided by the adhesive tissue that holds the layers of lamella together. Thus, much less energy is required to separate one layer of lamella from another layer (i.e. peel them apart), than would be required to cut through a lamella. The second factor is somewhat related to the first, and involves the stromal tissue response to photoablation. Specifically, for a given energy level in a photoablative laser beam, the bubble that is created by photoablation in the stronger lamella tissue will be noticeably smaller than a bubble created between layers of lamellae. The third factor is optical, and it is of interest here because there is a change in the refractive index of the stroma between successive layers of lamellae. This is due to differences in the orientations of fibrils in the respective lamella. When consideration is given to using a laser beam for the purpose of creating a corneal flap in a LASIK procedure, these factors can be significant.
In light of the above, it is an object of the present invention to provide a method for using a laser beam to separate lamella in the stroma of an eye which minimizes the heating of the stromal tissue. Another object of the present invention is to provide a method for using a laser beam to separate lamellae in the stroma of an eye that can be accomplished quickly in order to minimize the time a patient must fixate. Still another object of the present invention is to provide a method for separating lamellae in the stroma that avoids excessive trauma to the stromal tissue in the cornea. Yet another object of the present invention is to provide a method for separating lamellae in the stroma that is easy to perform and is comparatively cost effective.
In accordance with the present invention, a method for separating lamellae in the stroma of an eye requires focusing a laser beam between layers of the lamellae and photoablating tissue at the interface between these layers. This involves first locating a start point in the stroma. Preferably, this start point will be at a distance into the stroma that is approximately one hundred and eighty microns from the anterior surface of the cornea. As contemplated by the present invention, the anterior surface of the cornea can be identified using a wavefront sensor.
Once the start point is located, tissue at the start point is photoablated to create a bubble. The size of this bubble is then measured and compared with a reference to determine whether the bubble was created within a lamella or between layers of lamellae. If the bubble is created inside a lamella, subsequent bubbles can be created at different points in the stroma until the resultant bubble size indicates that photoablation is occurring between layers of lamellae. An ellipsometer is then used to detect a birefringent condition in the stroma between these layers of lamellae. Specifically, this birefringent condition will result from a change in the orientation of fibrils in the respective lamella, and will be indicative of the interface between layers of lamellae in the stroma. Further, it happens that from layer to layer of lamellae there will be a birefringent change that is manifested as a change in phase of about one half degree. Recall, the thickness of the lamellae is around two microns. The importance of all this is that the detection of a birefringent change will indicate a change from one layer of lamellae to another. Thus, it can be used to establish and maintain a focal depth in the stroma,
The photoablation of tissue along the interface between layers of lamellae in the stroma begins by focusing the laser beam to a focal point at the established focal depth in the stroma. Initially, the laser beam is set to operate at an energy level that is slightly above the threshold for photoablation of stromal tissue (i.e. above approximately one and one half microjoules for a ten micron diameter spot size). For example, the initial energy level that can be used for the laser beam may be around five microjoules for a ten micron diameter spot. In any event, whenever the laser beam is activated, there will be a photoablative response from the tissue that results from the particular energy level that is being used. Importantly, this photoablative response will vary according to the energy level of the laser beam, as well as the nature of the tissue that is being photoablated.
As intended for the present invention, the photoablative response is measured as the diameter of the gas bubble that is created in the stromal tissue during photoablation. This photoablative response is then compared with the reference value mentioned above to determine whether the initial energy level is sufficient for further operation. For the purposes of the present invention, this reference value is chosen to correspond to a hypothetical gas bubble in the stroma that, as a result of photoablation, would have a diameter of approximately fifteen microns. Depending on the difference between the photoablative response and the reference value, the energy level of the laser beam will either be held constant, or it will be changed. For the present invention, the change in energy level will be between a relatively low energy level (e.g. approximately five microjoules per ten micron diameter spot size) and a relatively high energy level (e.g. approximately fifteen microjoules per ten micron diameter spot size).
A condition wherein the photoablative response is greater than the reference value is indicative that the photoablation of tissue is occurring in the weaker tissue that is located at the interface between layers of lamella, rather than inside the lamella. Accordingly, further photoablation is accomplished by maintaining the initial energy level of the laser beam at the relatively lower energy level, and moving its focal point at the focal depth between the layers of lamellae. As this is being done, the ellipsometer can be used periodically to ensure the photoablation is being done at the same interface between lamellae. This continues as long as this condition persists. On the other hand, when the photoablative response becomes less than the reference value, the indication is that the focal point is no longer located between layers of lamellae. Thus, the energy level needs to be increased to a higher energy level. Also, the focal point needs to be moved until the photoablative response is substantially greater than the reference value. At this point, i.e. when the photoablative response becomes substantially greater than the reference value, the indication is that the focal point is again between layers of lamella. The energy level of the laser beam is then returned to its former lower value. Also, if desired, the focal depth can be verified by the ellipsometer and adjusted as necessary.
In the operation of the present invention, the energy level of the laser beam is altered in the above manner to follow the interface between lamella, and it is guided to create a flap from the corneal tissue. Specifically, the focal spot of the laser beam is moved within a boundary that can be generally defined by a first edge and a second edge. More specifically, to create the flap, the first edge should be a substantially straight line between a first point and a second point. The second edge can then be a curved line between the first point and the second point with the curved line having a radius of curvature around the optical axis of the eye of about four and one half millimeters. Further, this curved line should be centered approximately on the optical axis of the eye and extend through an arc of about two hundred and seventy degrees.